Terephthalic acid is an organic compound with formula C6H4(COOH)2. This colourless solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. World production in 1970 was around 1.75 million tons. By 2006, the global demand for purified terephthalic acid (PTA) had exceeded 30 million tons. There is a smaller, but nevertheless significant, demand for terephthalic acid in the production of polybutylene terephthalate and several other engineering polymers. In the research laboratory, terephthalic acid is popularized as a component for the synthesis of metal-organic frameworks. The analgesic drug oxycodone occasionally comes as a terephthalate salt; however, the more usual salt of oxycodone is the hydrochloride. Pharmacologically, one milligram of terephthalas oxycodonae is equivalent to 1.13 mg of hydrochloridum oxycodonae. Terephthalic acid is also used as a filler in some military smoke grenades, most notably the American M83 smoke grenade, producing a thick white smoke when burned. Due to its wide applications, many methods for its manufacturing and purification are disclosed.
Conventionally, terephthalic acid is produced by the wet oxidation of para-xylene in acetic acid medium using cobalt and manganese acetates as a catalyst and hydrogen bromide as a promoter. However, along with the main oxidized product, for example terephthalic acid, various other intermediates and side products are also formed during the manufacturing thereof. These intermediates and side-products remain soluble during the reaction for complete conversion. The terephthalic acid thus produced by the wet oxidation of para-xylene traps some of the intermediates such as 4-carboxybenzaldehyde (4-CBA).
However, in order to use terephthalic acid as a starting material, for example, for the preparation of polyethylene terephthalate, the content of 4-CBA is recommended preferably below 100 ppm. Therefore, reducing 4-CBA impurity, particularly up to 25 ppm, is very important for further use of terephthalic acid. 4-CBA, if it exists in large quantities in terephthalic acid, acts as a chain terminator during the PET polymerization process, and hence desired PET molecular weight may not be achieved. Conventionally, crude terephthalic acid is subjected to hydrogenation to convert 4-carboxybenzaldehyde into p-toluic acid and subsequently p-toluic acid is separated.
Existing Knowledge:
US20100174111 discloses the use of imidazolium chloride ionic liquid for separating terephthalic acid and to deal with the removal of 4-carboxybenzaldehyde (4-CBA) as an impurity. However, the aforementioned patent application does not disclose reduction in impurity after reconstituting terephthalic acid in stages, though there is a mention of a stage wise process for reducing the impurity.
Further, US20120004456 discloses the use of solution comprising mixture of imidazolium based ionic liquids with solvent including water, carboxylic acid, and alcohol for the separation of terephthalic acid. Although, the use of ionic liquids as well as molecular solvents particularly alcohols is disclosed for the separation of terephthalic acid in the above cited documents, removal of various intermediates and side products that are formed during the manufacturing of terephthalic acid is not disclosed in the aforementioned US patent application. Also these disclosures use ionic liquids as solvents and use excess quantity to dissolve terephthalic acid, thereby increasing the cost of the process.
There is, therefore, a long felt need to provide a simple, safe and an economic process for separating aryl carboxylic acids which considerably consumes less time and requires optimal amount of solvents as compared to the prior-art processes wherein the solvent is usually employed in bulk quantities.
Objects:
Some of the objects of the present disclosure are as follows:
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
Another object of the present disclosure is to provide a process for separating aryl carboxylic acids from a mixture comprising plurality of aryl carboxylic acids, intermediates and side products using ionic compounds.
Still another object of the present disclosure is to provide an ionic compound employed for the separation of aryl carboxylic acids.
A yet another object of the present disclosure is to provide an ionic compound employed as a solvent for the separation of aryl carboxylic acids.
Further object of the present disclosure is to provide an ionic compound employed as a solvent for the separation of aryl carboxylic acids from a mixture comprising plurality of aryl carboxylic acids, intermediates and side products, wherein the concentration of the intermediate products, particularly 4-carboxybenzaldehyde is reduced below desired ppm level.
Still further object of the present disclosure is to provide a simple, safe, efficient and economic process for separating aryl carboxylic acids.
Other objects and advantages of the present invention will be more apparent from the following description, which are not intended to limit the scope of the present invention.
Definitions
As used herein the term “liquefaction temperature” in the context of the present disclosure refers to a temperature at which a clear liquefied composition is obtained by heating together an ionic compound and an aryl carboxylic acid.
As used herein the term “precipitation temperature” in the context of the present disclosure refers to a temperature at which pure crystals of least soluble aryl carboxylic acid separates from a clear liquefied composition which is obtained by heating together an ionic compound and aryl carboxylic acid at a pre-determined liquefaction temperature of said aryl carboxylic acid.